IME, it's a pretty aggressive spreader (for a clumper) under the right conditions and normally grows in a very dense cluster of canes. What you have is a pretty skimpy plant for even a 1G container, let alone a 5G!! I hope you didn't pay a full 5G price for that rather paltry showing.

Once established, it should fill in. And the newer culms will be thicker and more vertical. 3.5' apart is a very generous spacing for this bamboo - I would have planted closer :-) Also needs a pretty mild climate, experiencing a lot of winter dieback in zones below 8.

I live in zone 9b so kinda warm. Darn! I knew when I saw the plants in green acres that I shoulda bought from there instead. I only bought 4 plants, some are taller, about 12'.